Oh my god. I would have frozen too. I mean, in my head I can see myself playing it cool, just sort of walking by him, not really interrupting his conversation, but just tapping him on the shoulder and going "Love your work!" before throwing him a smile and going on my way. This has worked ONCE in my entire history of being fangirly (with Kaia Wilson, the insanely hot backup singer that Amy Ray had on her solo tour this time around), but the odds of it working with Tennant would be slim to none. I'd probably start off well with the shoulder tapping thing, and then trip over something and go "SAVE ME DOCTOR."